text:
How can a passenger have opened an emergency exit on an Airbus A321 in flight?
Today it has been reported that a passenger managed to open the emergency exit door of an A321 in-flight. According to the report the aircraft (performing flight OZ-8124 from Jeju to Daegu in South Korea)  was descending towards Daegu when a left hand emergency exit opened, causing injuries to 9 passengers. The police arrested a man who was sitting next to the emergency exit and said he had been touching the lever.
I was under the impression that these exits were plug type doors, and so they are firmly pressed into their frame by the pressure difference between the cabin and the outside. Furthermore, I assumed some kind of lock (pressure lock, or something electromechanical) would prevent operating the door handle in-flight.
How is it possible that a such a door opens in flight?
Note: I am seeking to understand the mechanics of the emergency exit doors of the A321, not looking for speculations about what may have happened in this specific case.

Maybe the aircraft was already under the height at which the pressure equalizes between inside and outside? This height is 8000ft.

@U_flow cabin pressure is gradually increased during descent, so the cabin altitude also lowers below the ~8000ft that you have during cruise.

@U_flow, Bianfable is right; check out the chart in this answer.

Huh, good that I did not formulate this as an answer :)

Are we sure it wasn't an overwing exit? I don't think you would typically have a passenger being close to an actual exit door during that phase of flight (at least on the aircraft upon which I have flown). It wouldn't be the first time the media has gotten an aviation story wrong.

@RetiredATC The aircraft involved in this incident (HL8256) does not have overwing exits (see e.g. pictures here).

@RetiredATC Many aircraft have full size emergency exit doors directly in front of a row of passenger seats with no bulkhead. This is especially common on widebodies, but it's also the case on some narrowbodies such as the 757 and, as in this case, the A321. There's usually a bulkhead between the L1/R1 doors and the first row of passenger seats, but often between between L2/R2, L3/R3, etc. and the row of passenger seats immediately behind those doors. Passengers can egress faster through full-size doors, so larger aircraft often have those instead of overwing exits.

@RetiredATC The 757 and most widebody aircraft even commonly board through the 2L door or even both 1L and 2L. U.S. airlines don't generally board through 2L on the A321 due to its proximity to the #1 engine (don't want jet bridge operator accidentally bumping into the engine,) but, as I recall, some European airlines do.

So uh, are they going to put locks on the doors or what?

@Mazura that would severely compromise the "emergency" nature of the door operation

According to the FlightGlobal article you linked, the L3 door was opened (the emergency door behind the wing on the left side). All 8 doors on the A321ceo are plug type doors that cannot be opened when the cabin is pressurized. (Note that the A321neo has a cabin flex option for different door layouts)
I only have access to an A320 FCOM:

The aircraft has four plug-type doors that open outward and forward. There are two of these on each
side of the fuselage (two forward, two aft).
They can be operated from inside or outside the aircraft. Normal operation is manual, with hydraulic
damping.
Each door has features that tailor it to emergency situations:

An escape slide stowed in a container attached to the inboard lower side of the door
A damper actuator that limits door travel in normal mode, but in an emergency acts as an actuator
for automatic door opening
A slide arming lever.

(Airbus A320 FCOM - Doors - Description - Passenger Doors, emphasis mine)
The following YouTube videos show old Airbus training videos for the A321 and essentially say that the Type C emergency doors (L2/3 and R2/3) are identical to the main doors, except for size (only L2 is larger) and emergency escape slides:

Operacion Type C door
A321 EMERGENCY DOOR OPERATION

The aircraft must therefore have been depressurized at the time of the incident. During descent, the cabin pressure targets destination pressure +0.1 PSI:

During descent, the controller maintains a cabin rate of descent, such that the cabin pressure is
equal to the landing field pressure +0.1 PSI, shortly before landing.
The maximum descent rate is 750 ft/min.

(Airbus A320 FCOM - Air Conditioning and Pressurization - Pressurization - System Operation)
Shortly before landing, the cabin pressure is therefore only slightly higher than the outside. According to the FlightGlobal article, the door was supposedly opened at 800 ft. Together with the pressure from the nitrogen bottle that assists the opening if the door is armed, this should make it possible to open the door.
Note that there are no centrally controlled locks on the doors. The door locking is entirely mechanical and controlled by the door handle. Instead of pressure locks, there is a Cabin Pressure warning light that warns the crew on the ground in case of residual pressure difference:

Red Light: flashes when one or both engines stopped, the slides are disarmed, and the cabin differential
pressure is above 2.5 Hpa.

The fact that there is a warning light above 2.5 hPa (0.0025 bar, 0.04 psi) suggests that there is a range of pressures where the door can be opened and it violently slams open. But I could not find any number of the maximum pressure difference at which this can happen.

rough back of envelope numbers: 0.1 PSI * ~1m^2 of door surface area -> ~150 lbs of force to open the door. Seems possible with some mechanical advantage and the nitrogen assist you mention.

I remember back in the day when I worked part-time as a baggage handler, we sometimes had to open the rear door (mostly of Boeing aircraft)  [we were sometimes rostered on to the cleaning crew if they were short]. We were always trained never to open the door until we received a signal from the cabin crew that the door was safe - we we told it would otherwise automatically blow open. I assume this was the nitrogen as there was no pressure difference (the front door was usually already open)

CNN reported that the passenger "opened the door when the aircraft was about 700 feet (213 meters) above the ground and about two to three minutes from landing in the city 150 miles (240 kilometers) south of Seoul."
At 700 feet, there is not enough pressure-differential between the inside and outside of the plane to make opening a door difficult.

As I learned in this question: There is always a little pressure difference in flight. See the discussion and link under the question.

@U_flow Yes, a little, but at 700-800 feet it is just that - little. From sea level to 700 feet is only about 0.3 psi, for example. For a Type C door, that would require about 400 pounds of force to open. Still not a feather, but possible (especially given the lever arm involved.) Whereas in cruise it's more like 10 psi differential, requiring over 14,000 pounds of force.

According to this video,

, the aircraft was on final approach & was at about 600ft.The door was armed & therefore its opening was assisted by a compressed nitrogen device. The nature of this door is that although it opens forward, it does so remaining parallel to the fuselage, a bit like a common delivery van side door. The event was "interpreted" by the aircraft system as a valid emergency exit & the escape slide was also deployed. The slide was ripped off by the slipstream. I have no idea if this door should normally be inoperative without weight on the undercarriage, although I could imagine that it is thought that the aircraft might not necessarily be rightside-up, or even have its undercarriage extended, when emergency exit is required.

Good to see that  Juan Browne 'Blancolirio' has at least one aviation.stackexchange.com tab open in his video. Wouldn't be surprised if his research took him to this question :-), he is using the video linked from Bianfable's answer to explain how the doors work.


text:
Why does the Boeing 787 engine nacelle exhaust have such an unusual shape?
Why does the Boeing 787 nacelle have this unique shape at the exhaust:

I haven't seen it on any other airplanes, does it serve an aerodynamic purpose or help engine efficiency or is it just to look pretty?

Well, it does look pretty, doesn't it?
Boeing wanted to reduce the noise generated because of jet blast of the engines. Many airports around the world are implementing new noise regulations.
As mentioned by Boeing:

To combat the sound of jet-blast from the rear of the engine, Boeing,
  General Electric, and NASA developed serrated edges called chevrons
  for the back of the nacelle and the engine exhaust nozzle. The
  chevrons reduce jet blast noise by controlling the way the air mixes
  after passing through and around the engine.
The acoustic liners and chevrons are such effective noise suppressors
  that several hundred pounds of sound insulation may be eliminated from
  the fuselage.

As NASA was involved, they state (my paraphrasing):

Chevrons are the sawtooth pattern seen on the trailing edges of some
  jet engine nozzles. As hot air from the engine core mixes with cooler
  air blowing through the engine fan, the shaped edges serve to smooth
  the mixing, which reduces turbulence that creates noise.
... The new Boeing 787 is among the most modern jets relying on
  chevrons to reduce engine noise levels. It sports chevrons on the
  nacelles, or fan housings.

Looks like the goal is a phase shift in the exhaust for destructive interference.

Turbulence is what creates noise, and turbulence is inherently random. Phase shift can only help with non-random sounds.

@romkyns turbulence is chaotic, not random. For example, in BL turbulence in buffer layer has some "predictable" behavior (which allow to create numerical turbulence model), something that a random pehonomena would have not allowed.

@TrebiaProject. I suppose "chaotic" is a better term, but "phase shift" will still not help to cancel out a chaotic effect, agreed?

Yes, of course. Actually the fact of having a chaotic flow motion ends in a broad band noise making phase shift impossible as no defined tone is available. So further supports your argument.

I think both answers are correct, but I would like to spend some time describing the phenomena that chevrons try to reduce so that I can complement the answers already provided.
So, what is happening in an engine? We have a hot gas at high speed leaving the engine core, and another gas at higher speed than the external air but much slower than the core, on top, and we have got the external airflow. Actually I am missing one that I will explain later on.
OK, so between airflows with different properties (especially different speed) there is a layer where both airflows mix to create a different one with average properties (saying average is a simplification), the mix in a region called "mixing layer". Unfourtunately, in typical engines, the way both layers mix is turbulent. Here is a typical image of a simulation of a mixing layer.

This turbulent mixing creates sound. Actually is easy to see a similar phenomena in a flute. If you open it, you can see that when you blow you are introducing air that reaches a cavity where sound is produced (actually, there is another phenomenon, resonance, but I would like to illustrate that turbulence creates sound). Another example is when you wave a sharped object in air (like a blade); if you do it quickly enough, you will hear a sound.
That exact phenomenon happens around engines, but the sound is louder. Why? Because the difference between speeds is really high. A glider seems silent although the wings act as blades... but the difference in speed is not so high.
So, what we know now is that the smaller the difference, the smaller the turbulence, the quieter the sound. Those chevrons you see in Boeing 787 actually have that objective. They are actually creating another layer of fluid at a intermediate speed between the air passing the external part of the engine (fan secondary flow) and the external flow, so that the jump of properties is smaller, turbulence gets reduced and sound is quieter.
So, to show you the flows in an engine I took your photo and introduced some arrows:

Basically, the blue air coming for the external part of the nacelle mixes with the air coming from the fan (red arrow). Having the chevrons means that some parts are mixed earlier than others, creating positive distortion in the mixing layer like "creating a third flow". The green arrow shows the air from the engine core, but there is another air flow between engine core and the fan, the violet arrow.
What is shown in the previous answers for Conway and JTD-8 engines are devices using the same principle, but they are actually not the same. In previous jet engines there was not a big secondary flow from the fan as nowadays and the mixture was directly between engine core and external flow, so it is the same principle, but the huge difference between speeds created the need of a more aggressive mixture. This device is called hush kit.
Finally to answer your question, there is no aerodynamic benefit; there is a penalty. The benefit is aeroacoustic.
There is an indirect benefit, though: The amount of acoustic damping material needed to get the cabin quiet enough is reduced. That saves weight, and therefore fuel.

Do you by any chance have a simulation of the mixing layer with a design like this so that we can visualize the difference?

No sorry, actually the state of the art is far from being able to simulate this kind of devices (without simplifications)

@MichaelT that is actually what I love from the industry and how, globally, one effect has a overall effect on the airplane. Nice comment! Thanks!. However the effect is curretly limited by compressor noise, with the current improvements in jet noise, compressor noise is becoming of the same order.

@MichaelT Good point - I added it at the end of the answer.

Actually, there were similar tricks used on Boeing aircraft before to reduce jet noise.
The Rolls-Royce Conway (as used on the Boeing 707) had a scalloped exhaust which improved jet mixing and reduced exhaust noise. Since the Conway was also the first operational bypass engine, the lower exhaust speed of this design helped to reduce noise already. At the time of development the engineers expected the engine to be installed in wing roots, and the bypass ratio was only 0.25, however.

Also the JTD-8 of the Boeing 737-100 used a similar silencer design:

In all cases, the idea is to increase the surface between the hot, fast exhaust gas and the ambient air and to stretch out the noise-intensive mixing process.
See also this question for more information.

I get that for the exhaust, it should be carefully shaped in order to get the correct flow.  I was curious though because I had never seen it on the nacelles like that though!

@Lnafziger: Yes, the chevrons at the end of the nacelle are indeed new. Only the principle hasn't changed. Sitting between fan and outer flow, they provide only a little noise reduction, but they look cool and show that GE does something. In that, they are similar to winglets.

The main purpose of the chevrons in the engines is to reduce the engine noise.
Most of the civil airliners use high bypass turbofan engines, which produce significant amount of noise especially at high thrust conditions. Aircraft (engine) noise is especially critical during the takeoff and approach phases as it affects the people in the area around the airport and also the ground crew. Reduction of jet noise has been one of the focus areas of civil aircraft manufacturers since the (failed) SST program.

Source: adg.stanford.edu
As can be seen, the engine noise is perhaps the most important contributer for the total aircraft noise.Thus reduction of engine noise becomes paramount. Most of the high bypass turbofan engines have two flow regions- the central core and the surrounding fan. These regions normally don't mix with each other in the short nozzle.

"Airbus Lagardère - GP7200 engine MSN108 (1)" by Dr Brains - Own work. Licensed under CC0 via Commons.
The schematic below shows the operation a typical high bypass turbofan engines used in civil airliners.

"Turbofan operation" by K. Aainsqatsi - Own work. Licensed under CC BY 2.5 via Commons.
Usually, high bypass ratios reduce noise. The main source of the jet noise is the turbulent mixing of the shear layers in the engine exhaust. These shear layers contain instabilities that lead to highly turbulent vortices that generate the pressure fluctuations responsible for sound.

[2012, V. Parezanovic] Source: http://www.berndnoack.com/
One of the was to reduce noise is to mix the core and bypass gases effectively. NASA carried out research in this area and evaluated a number of different configurations.

Source: NASA- Innovations in Aeronautics
These studies revealed that though the lobed mixers reduced noise better, the associated thrust loss was more and the chevrons offered the optimal solution. Two solutions, tabbed and chevrons were then (simulated flight) tested in various configurations.
These test showed that the configuration with chevrons in fan nozzles only caused the least reduction in thrust coefficient. This probably was the reason this was selected.
In addition to this, research was carried out in developing systems which would optimize the chevron 'immersion' into the jet flow based on the flight condition. Shape memory alloys activated by heat were developed which would allow for full chevron immersion in jet flow during high thrust requirements (for e.g. during takeoff) and not immersing it during cruise where the thrust efficiency is of greater importance.

Variable geometry chevrons for immersion control (AIAA 2006-2546) (Boeing image)

Other things being equal (that is, for the same thrust), "high bypass turbofan engines" have less noise than any standard alternatives (that is, turbojets or turboprops) even without any special measures. What I want to say is that while they do "produce significant amount of noise" which we want to reduce, it is not specific to high-bypass but rather to jets/turbines in general.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
Touch-and-go methodology in high-performance singles
I flew a high-performance aircraft for the first time recently (SR22T) and nearly ground-looped the thing during a touch-and-go because the instructor gave me absolutely zero pre-flight instruction (my brother is taking lessons and I'm certificated with ~100h in a 172, which I made abundantly clear).
There are two contributing factors in my mind:

I was making copious use of the nose-up trim during final for a zero control-pressure at approach speed. The landing was butter, but as I advanced the throttle the thing literally jumped off the runway and I panicked as I tried to keep it in ground effect without adjusting the trim.
I advanced the throttle quite aggressively which caused an enormous amount of yaw, where during my panic, instead of rudder, I tried to steer the thing like a car with the obvious result. Luckily we were off the ground and avoided a worse-case result, though  I'm pretty certain that the AFCS(a) saved us from stalling out in the chaos.
(bonus) The instructor quite obviously abdicated his responsibility to the safety of his passengers and the aircraft. Though, I have enough experience that I should not have flown without a briefing.

It was terrifying and it has made me apprehensive about taking the controls of such an aircraft again.
What briefing or process should the instructor have used that would have ameliorated this outcome? How should you handle trim on such an aircraft for a touch-and-go?
(a): The SR22T I flew had a Garmin G1000 with an Automatic Flight Control System, which includes a feature sometimes referred to as "envelope protection" that is active even with the autopilot disconnected. One aspect of this protection is that it will (for example) reduce AoA like the stick pusher in a commercial aircraft to keep it within its safe flight envelope, preventing a stall.

Re point number 1, the question would be improved by explaining specifically which control surface(s) you found yourself trimming on final.  Elevator?  Rudder?  Both?  Etc.  Also re "though I'm pretty certain that the AFCS saved us from stalling out in the chaos" -- can you elaborate just a bit more, and spell out what AFCS is?  This sentence will be a complete mystery to many readers unfamiliar with this specific aircraft.

Good point! I've clarified that it was only elevator trimmed to approach speed.

Upvote for use of the word "ameliorated".  ;)  (you made me look it up...)

Yeah you were very poorly briefed. With a tractor airplane with the tail in the propeller wash and the engine thrust line close to the C of G, trim speed can drop significantly with power. The 172 will do this as well, but the effect is a lot more subtle. I can imagine full power in a beast like the Cirrus lowers the trim speed by 5-10+ kt, just from the increased propeller wash, and that on top of torque effects can be a handful.
The proper way to do touch and goes with an airplane like that is to retrim on the roll to a setting that is close to the desired one at full power, which you can do while you're reconfiguring flaps for the go.
I've done touch and goes in CRJs, flying them in a circuit/pattern like a light plane, and the procedure once down is to maintain the centerline while the other pilot configures trim and flaps, and when they are set stands the thrust levers to around vertical, enough to get the lift dumpers to retract, confirm everything is good, set climb thrust (not TO thrust on T&Gs), and off we go. Retrimming is a critical part of the procedure.
In the case of the Cirrus, you're going to do that yourself obviously. One suggestion is to do some experimentation on the ground to see how long it takes to retrim from the approximate approach speed trim value to the take off trim value. Count it out, one-thousand-two-thousand-three-thousand or whatever, and when you do touch and goes, as you are resetting flaps hit the trim for the three count while you are doing it, with a quick glance at the indication to confirm it's reasonable. You'll be close enough, then when you lift off it's light control pressures to achieve the target speed and blip blip blip with the trim to get it exactly where you want it.
If you are going around while still airborne, it's the same thing really; if you have a rough idea of what it takes to retrim for full power climb, just develop the instinct to give it a couple of shots of ND trim as you are feeding in the power. As well as developing the instinct to feed in the rudder you know is going to be required.
And yes you should be getting in the habit of adding and reducing power reasonably slowly, 2-3 seconds from idle to full throttle, especially with high performance machines with turbocharged engines like that, to allow the turbo waste gate controls and propeller governor to keep up without overshooting. It's just good engine management practice. The exception is when you need power right now to stay alive.
It's basically a matter of learning the airplane's behaviour and using that knowledge to stay ahead of it. A proper briefing and probably a demo T&G by the instructor on the first try would've helped a lot. But don't be intimidated by the thing.
Treat it like a horse, which has to be shown who's boss if you don't want it to take you back to the barn when it feels like it. Take charge. Command it. A lot of it is psychological.
The way to get there is knowledge and practice, and sometimes, experimentation done at a safe altitude as well. And for sure, better training.
Drill the airplane's procedures in your mind, over and over again. If you have a Cirrus computer simulator, use that to drill and drill and drill, practice practice practice, until you can't stand it. When the time comes, your hands will fly around automatically without thinking, and your brain will be free to concentrate on actually flying the thing. Computer sims are a very powerful tool for that. Then get back on the horse.

"The proper way to do touch and goes with an airplane like that is to retrim on the roll..." --this really surprises me.  Even during single-pilot operations?  Keeping in mind that depending on exactly when power is re-applied, sometimes the amount of time spent with the wheels on the ground can be rather brief--

When doing T&Gs, (or "circuits" in Canada) you will be rolling for the time it takes for landing flaps to be retracted to takeoff flaps. You are normally on the ground for 3-5 seconds at least, depending on how long it takes to reset the flaps.  In a C-150, you don't have to bother with the trim and can take care of it during the climb. If there is a major trim change between the two configs, then yes you should use the interval to reset trim to the normal takeoff range. Otherwise you takeoff again and have to hold 30lbs of forward pressure as you frantically re-trim. Quite dangerous.

Excellent point on trim speed changing, excellent answer!  I might have to tweak mine a bit...

EDIT:  Read John K's answer regarding trim speed changing with power.  I will modify my answer later to correct my statements regarding this...
Caveat:  I have never flown the CR22T, and have never done a touch and go in a piston single over 180hp.  I have some number of T&Gs in the Cessna 208 and T-34C, but there is some lag in the turbine spool up so I am not sure how they compare.  I have done thousands of T&Gs in jets.
ELEVATOR TRIM:
In most aircraft the approach speed is pretty close to rotation speed.  Your technique of trimming for final approach speed is correct, and I don’t recall in any aircraft ever being taught to re-trim during a T&G.  Therefore I don’t recommend changing anything in your habit pattern, just know that unless you leave it at idle and decelerate significantly during the ground roll that with extra power you will get airborne again very quickly.  As you noticed…
Control pitch on the go, but don’t fight to hold it in ground effect as long as your speed is under control.  Because you weren’t in ground effect on final at the speed the airplane is trimmed for, so you know the airplane will fly.  That might mean a higher nose attitude than you are comfortable with, but you will get used to it.  And even come to like it!
Of note, it took some adjusting on my part to get used to light piston single GA T&Gs, letting the nose fall through, maybe changing flap configuration, carb heat in, advancing throttle slowly…  In Navy jets you don't mess with anything, you literally “touch” and go, full power and backstick as soon as the main wheels hit.
YAW:
It would seem that a HP piston single might have a faster response than what I have flown, but yeah… you need to be ready for it.  WWII fighters could put you right on your back as I understand it.  Feed power to it a bit slower next time, and counter with rudder.  Basic stuff, and I know you know it, but that leads me to…
INSTRUCTOR/YOUR REACTION:
Yes, your instructor could have “ameliorated” the situation with a better pre-flight briefing on what to expect, and/or ridden the controls a bit.  Hard to say because I wasn’t there, but saying they abdicated responsibility for safety sounds a bit harsh.  It’s difficult to anticipate how a student might react, and perhaps with your experience they presumed you would handle it better.  It sounds like you overreacted, gave both of you a bit of a scare, but hopefully you had a thorough debrief so that both could learn from the experience.
Your story reminds me of years ago when I traded in a tired 4Cyl Fox Body Capri for the same chassis in a newer Mustang GT with a V8.  On my first test drive I was just trying to accelerate briskly to compare power to what I was used to.  In doing so I unintentionally lit up both back tires and started drifting sideways before backing off the throttle.  It startled me, but as soon as I learned to respect and control the power I loved it.
These sort of events definitely get the heart pumping, but you walked away unscathed and no mishap on your record so please don't be discouraged.  And I wouldn’t dramatically change anything in your methodology, just be more cautious until you develop a feel for it.

That is a really great point about being trimmed to fly, hadn't thought of it that way! Compared to my 172 where the throttle is more of a suggestion, that plane is an absolute rocket with a 78" composite prop. I overreacted without a doubt, the stall horn was chirping from my panic porposing the plane trying to level when it felt like we were going almost vertical. Watching some T&G instruction online, the instructor was forcing the student to advance the throttle while counting to 5. There was no debrief, so I came here. Thanks for the perspective, next time maybe I can enjoy the power.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why do some non-towered airports have class E airspace from the surface up?
I've noticed that when it comes to non-towered airports, some are in class G airspace and others have class E airspace starting from the surface. At first I thought the airports with class E at the surface might be in that configuration because they have precision approaches, but then I found exceptions where the airport has class G at the surface along with a precision approach. Is there a particular reason some non-towered airports are in class E while other are in class G?

I have a hard time understanding why a relatively large, busy airport like CVO is not Class E to the surface.  Surely all the required conditions are met-- or, maybe not...

This answer is specific to the United States (heck, I don't know if this is an even an issue elsewhere in the world).
The airspace classification actually has nothing to do with whether or not a particular airport has a precision approach. It is based on two factors: communications capability and weather observations.
For a non-towered airport to have Class E (surface) airspace, ATC must have communications capability with aircraft down to the runway surface. Second, weather observations must be provided by a Federally certificated observer and/or Federally commissioned weather observing system. Reference: FAA JO 7400.2K, paragraphs 18-1-1 and 18-1-2.
If one (or both) of these conditions are not satisfied, the surface airspace will be class G with a class E floor of 700 ft. AGL if there is an instrument approach into the airport and 1200 ft. AGL if not.
Now, here's a twist: if the airport has a part-time control tower, the airspace may become Class G when the tower is closed, even if both communication and weather reporting requirements are satisfied. Be sure to check the Chart Supplement (fka A/FD).

How can you confirm if a non-towered airport is E versus G? Does the FAA maintain a list?

This information is depicted on VFR charts. If you look in the chart legend, there is a notation that states 'Class E Airspace exists at 1200' AGL unless otherwise designated as shown above.' This means that an airport's airspace will be Class G unless there are graphics that indicate Class E starts at a lower altitude (i.e., 700' AGL or at the surface).

Can you give a reference for your last point re: after control tower hours? AIM 3-2-5 says "when the control tower closes, the airspace revert back to either class E, or a combination of Class G and class E depending on the chart depiction and the description in the A/FD" but mentions nothing about weather observations.

@DavidDeVine, the last point was poorly worded as it is not a universal condition. I've updated it accordingly. Classification as controlled airspace (i.e. Class B, C, D, or E) is not automatic, thus an airport with a part-time control tower may (or may not) be controlled airspace when the tower is closed, even if the conditions for Class E surface area are met.

@DavidDeVine -- it's interesting that AIM 3-2-5 is no longer worded that way, there is no reference to "a combination".  There was a major rewrite in 2016, I wonder if your reference came from before 2016?  I guess the "combination" idea was in reference to the vertical structure of the airspace, i.e. Class G up to (normally) 700' or 1200' AGL and then Class E above-- as opposed to the alternative situation of Class E all the way down to the surface--

It's also interesting that as long as the airspace has no surface-level Class E extension, there is no way to tell from the chart alone which of these two possibilities is the case for part-time Class D airspace.  You have to check the Chart Supplement to see whether the surface-level airspace converts to G, or to E, when the tower is closed.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is this thing between the engines on the F-14 Tomcat?
On the F-14 as you can see from a picture there is some exhaust like thing in between engines. What is this?

Related drawings.

This is the line for dumping fuel in an emergency. If the F-14 needs to land quickly but is too heavy for landing, fuel can be pumped overboard through this pipe. To the left of it is an antenna, and the rod below the fuselage is the arrestor hook.
The Australian F-111s use the same arrangement to great effect during flight displays.
From the F-14 Walk-Around Photo Index Page:

Back end of the "beaver tail".  The red hole is the fuel dump point.
  The beige item is the rear ALQ-126 antenna, paired to the ones on the
  wing glove area.  This antenna, like the forward ones, was introduced
  with block 110 F-14As and all subsequent aircraft.  Also visible is
  the hook portion of the arresting hook.

Cool. From its appearance and location I would have guessed it was an APU exhaust. Do fighters even have an APU?

@TomMcW: Yes, most modern ones do. See here for a related question

The only thing I can add here, is that the two square holes on the underside next to the tail hook, are for the chaff and flares.

Hello there and welcome to Aviation.SE. We're not like other forums; the answer box is only for answer. This post should be a comment because it does not answers the question. That said, I think it does add interesting information so I think we can have an exception here. Just remember, next time use the comment feature to comment (-:

From the picture holes look right above the hook. I wonder how they avoid hitting the hook.


text:
Which altimeter setting should I use?
In the Denver metropolitan area (where I fly), we have a large class B area around KDEN, with the associated approach control providing services to the surrounding airports (KAPA, KBJC, KFTG) as well.
Whenever I fly IFR and am in contact with Denver Approach, I am given the Denver altimeter instead of the altimeter of the closet airport which is usually (slightly) different.
I use the Denver setting while in contact with Denver Approach, since I've always assumed everyone else in contact with them are doing the same, and only switched to the destination altimeter after being handed off to the tower controller at my destination.  Is this considered the correct procedure?

Yes, that is correct procedure, except that if you're flying an instrument approach, switching to destination altimeter should be part of your approach setup "chart briefing" (which likely happens before you get handed off to the tower).
From AIM 7-2-2(a)(1) (emphasis mine):

[Set the altimeter] to the current reported altimeter setting of a station along the route and within 100 NM of the aircraft, or if there is no station within this area, the current reported altimeter setting of an appropriate available station. When an aircraft is en route on an instrument flight plan, air traffic controllers will furnish this information to the pilot at least once while the aircraft is in the controllers area of jurisdiction. In the case of an aircraft not equipped with a radio, set to the elevation of the departure airport or use an appropriate altimeter setting available prior to departure.

Unfortunately the AIM text doesn't directly address destination altimeter settings, but section 4-1-8 alludes to the fact that controllers must verify you have the best destination altimeter setting, either by telling you or by having you report that you have the current ATIS or the AWOS/ASOS broadcast ("the numbers" for the latter):

a. Numerous approach control facilities have established programs for arriving VFR aircraft to contact approach control for landing information. This information includes: wind, runway, and altimeter setting at the airport of intended landing. This information may be omitted if contained in the Automatic Terminal Information Service (ATIS) broadcast and the pilot states the appropriate ATIS code.
b. Such information will be furnished upon initial contact with concerned approach control facility. The pilot will be requested to change to the tower frequency at a predetermined time or point, to receive further landing information.

For instrument approaches under IFR, setting and checking the altimeter to the destination's setting should be part of your approach briefing checklist. My personal "checklist" includes the altimeter setting as part of the weather information when I get to the bottom of the chart and check ATIS/ASOS/AWOS numbers against the approach minimums.
Interestly enough, the FAA's Instrument Flying Handbook doesn't have much to say on altimeter settings at all (except for mentioning that keeping the setting up to date is important and that bad things can happen if you don't).

The important point here is that, as per the AIM, the DEN altimeter setting is valid within 100nm of the station. Although, I would imagine that if you're right along the front range, you'd get something closer


text:
Why do multi-engine privileges not propagate down to single-engine?
From this answer, privileges of single-engine and multi-engine are separated. A pilot who obtained his PPL on a single-engine and his CPL on a multi-engine is not permitted to exercise his commercial privilege on a single-engine airplane. To do so, he must past another CPL exam on a single-engine plane.
Why is this do? I see no reason why a pilot who can operate a multi-engine airplane safely is incapable of doing the same in a single-engine. Training in a multi-engine would include an all-engine-failure scenario, which would be similar in single-engine.

They (multi-engine certified) probably don't practice total engine failure, given its very rare, to anywhere the near the level of single engine pilot- the probably of single engine (for one engine) failure is higher than multiengine failure but lower than single failure for a multiengined plane.

Having taken both I can say the single engine is far more challenging. You really don’t take a multiengine much beyond it’s ability because an engine failure below vmc may not be recoverable. For the single engine you have to do semi aerobatic maneuvers that keep you on the edge of stall. In the twin you have to fly around and shoot approaches on one engine but once you figure that out it’s pretty easy.

Just like having a SEL Commercial with a MEL PPL doesn't mean you can operate a MEL commercially.
The certification requirements require the demonstration of particular maneuvers like a Chandelle which includes a slow-flight phase. Slow flight on a single is significantly different than slow flight on an MEL because of p-factor and aircraft handling. Same thing with lazy-8's
Another reason is engine-outs. Engine outs have to be demonstrated, and on a Multi, an engine out means you can limp it to your closest airfield. On a single, unless you are in gliding distance, you will be putting it down off-field.
The FAA also has different test standards for SEL commercial versus MEL commercial. Because you've demonstrated satisfaction on an MEL doesn't mean you are proficient in SEL operations for the same maneuvers, so the FAA makes you demonstrate it.

On a multi isn't an all engine out scenario practiced?

@curious_cat I believe so, but so is single engine-out, which I would say is a lot more common. All engine out is typically because of a shared failure, like no fuel or incorrect fuel and I say a rarer occurrence.

No we never do all engine out in a multiengine. It’s not part of the cerification standards.


text:
What prevents someone from hijacking a radio frequency mid-air?
For someone who wants to stay anonymous and probably terrorize the skies, what protective measures do we have?
Without even thinking too deep, I can imaging the following scenarios for someone who's in a mood to mess around.

Flying in a busy airspace, calling to "Cancel IFR" for someone else.
Abusing recklessly. Maybe stepping-on the frequencies infinitely.
Reporting fake traffic or fake weather conditions, causing re-routes
and delays.

While transponders are required in and around busy airspace, someone who really sets their target on screwing it up can REALLY cause a lot of harm, potentially fatal by flying without one for quite some time as a primary target on ATC's radar.
What are the current preparations to stop this from happening?

Comments are not for extended discussion; this conversation has been moved to chat.

This happened recently:  https://www.theregister.co.uk/2016/11/24/melbourne_man_arrested_for_broadcasting_fake_messages_to_pilots/

Some nice men with guns? (who will find you after the fact and drag you to jail)

This sounds like a “people problem”. That kind of problems is best solved with enforcement.
Source of radio broadcast can be localized and it is not particularly difficult. There are techniques for transmitting something secretly, but they need specific modulation. Rogue broadcast or jamming on simple AM voice channel will be trivial to triangulate.
So if somebody did it, the FCC or FBI would pull out appropriate equipment, hunt them down and throw them in jail. Broadcasting without license, or contrary to the license you have, is in itself a crime and they'd throw in charges for endangering, which are quite serious.
Now of course it would take some time to hunt the perpetrator down. However, the situation is not that dangerous. Basically it could be treated as communication failure. VFR flights would continue flying visually, IFR ones would follow last clearance and then as filed, landings can be cleared with a light gun. And then either the surrounding sectors would be OK, or the pilots would probably try to establish some alternate communication. Cell phones should work fine for small aircraft while airliners would probably use HF or Sat links to dispatch and ask the dispatcher to relay.

There was the odd incident years ago of a new TV set broadcasting a distress signal. They found that guy in short order (TV company replaced it for free). Then there was the kid caught swatting people. Took a lot longer to catch, but they did. Someone doing this wouldn't be doing it for long.

How are flights and towers authenticated? What if somebody pretends to be the tower and gives bad instructions to aircraft?

@Michael, they are not. However, it is a shared medium. The real tower will hear somebody impersonates them and send the police after them. It can also still broadcast itself, so it can fix the commands.

This is really the same as a failure caused due to unintentionally radiating electronics. I live near an airport and at least once a year there are reports about things like a broken alarm clock radio disrupting a frequency (or range) each day at 7 to 8 ....

@JanHudec: Would it be possible to send a strong, directional noise signal towards the tower and a strong fake signal towards the aircraft which overpowers the tower’s signal?

@Michael, possibly, but you would have to make the directional antennas, you'd have to find where the tower receivers actually are (there may be more than one) and then the police would go after you pretty quickly anyway. And then, the tower would still see what the plane is doing and would try to do various things about it, unless you also spoof the radar, which is far more complex.

@Michael, also the stronger your signal, the easier it is for authorities to track it.

Remember that directional radio signals are not point-to-point but point-along-a-line-to-another-point. Authorities just need to drive or fly around the control tower to find the direction your signal comes from.

@Snowman, also, don't imagine directional broadcast as a ray of light with no signal outside. Due to the wavelength involved, antenna do create that would be monstrous. No chance of moving it anywhere. Anything small enough that you can actually move it will have quite wide dispersion and side-lobes and there will be at least some signal almost anywhere.

I think it is a vulnerability, yes.  But it has worked so far, much like the "if hijacked, cooperate with hijackers and let things sort themselves out" doctrine that saved many lives until someone figured out how to game the rule (and cost as many lives as were ever saved).
Indeed, right now, we depend on these being rare cases, that can be dealt with in the methods used so far, which are surprisingly ham-handed: sending FCC radio trucks, civil air patrol or ham operators on transmitter hunts.  It could be a great deal more difficult if you had concerted efforts by quasi-state actors acting on a large scale - not one rogue transmitter but 20 in each of 10 regions, and the transmitters are moving.  Imagine an actor learned to perfectly mimic Kennedy Steve, and sat there on channel giving wrong instructions, or heterodyning key instructions.
And of course, if a quasi-state actor is involved, this could itself be part of a much larger scheme, such as simultaneously, 4 planes code hijackings while 19 cargo planes turn off their squawks entirely, all this to turn military eyes toward missing planes and away from their normal posts... So then somethiing else happens.  This is what you can expect modern war to look like in a highly technological and networked society.
For all the technology, human workload is still the limiting factor - so such nuisance attacks would be all about creating excess workload and distraction for the guardians to put them off their normal game, so you can slip something through that would be otherwise impossible.
A huge amount of this kind of cyber-war is who is making workload for whom.  You want the hackers to be the frustrated and overworked ones.
A solution for radio
Speaking of heterodyning, I was blown out of my seat by the Midway Southwest-Delta incident, where similar flight numbers acted on each others' instructions and -just like Tenerife- heterodyned over each other.  Seriously? Still?? SMH...
I've been carrying Hedy Lamarr's frequency-hopping tech in my pocket since 1997 (CDMA).  This allows a bunch - a whole bunch - of phones to share the same frequency block without stepping on each other, and it works.  Obviously, this also authenticates the transmitter, since voice wasn't free in 1997.
And duplex radio is nothing new, that's where you receive on frequency R but transmit on frequency T, and the repeater transmits on R anything it hears on T.   Combine them and it becomes a simple matter: the control tower transmits on one frequency, and planes use CDMA to talk to the tower.  Every plane has a separate virtual channel, with encryption used for identification.  When two planes talk at once, they both get through.  Assuming the control tower configures it as a repeater so you hear all traffic, you hear both KLM and Pan Am.  Even slicker, there are several possible ways to mute your own transmission, so KLM doesn't hear itself, they only hear Pan Am - and vice versa.  This would be the everyday advantage.
During an attack, ATC can mute "that guy" or can mute all traffic except identified traffic, and ask unidentified traffic to ident in some alternate way, like "please touchtone 5 7 8", and keep changing the "CAPTCHA" as it were.
Stepping on CDMA is hard, because all radios are stepping on each other all the time normally. So it would be hard to block the tower's transmissions outward, or planes' inward.

and cost as many lives as were ever saved [citation needed]

Before 9/11, pilots were told to cooperate with hijackers?

@Someone yes. I know it seems insane today. There was a well-tuned playbook for exactly how to deal with hijackers, that had worked well for decades.  People were aware suicide hijackings were possible but there wasn't consensus to change the playbook.

The reality is that there are stuck mics daily on ATC frequencies.  Pilots and controllers utilize guidance from the AIM and sometimes creative improvisation. There have been numerous cases of lone actors wreaking havoc with ATC in various countries and they are usually located with direction-finding and dealt with by police.
There is no strong countermeasure.  Company planes and carriers normally have alternative com equipment which has been used. Normally cellphones are impractical.
So a swarm of illicit VHF AM transmitters could wreak havoc but the peril is rather inconsequential.

Yes, in fact 9/10 of the preflight planning done for IFR flights is to cover the scenario where communications are lost. There are protocols to follow. If things get too confusing or blocked, people will follow those.

@quietflyer I bet it's a typo for "lone actor."

Range of transmission will limit damage, and the transmitter could be located fairly easily.  The offender will go to jail.

At 5 or 10,000 feet, not to many places to hide.  Basicly broadcasting "please, throw me in jail".

Just the rules and physics.

If I was flying and I heard over the radio something like "Flight 122, Jacksonville Center, Request cancel IFR" I could just respond with "Flight 122, Jacksonville Center, That was not a legitimate request, we wish to maintain IFR" and though my phraseology is wrong, I could reasonably assume they got the messages. I would continue my clearances and flight plan till it got worked out. It would not take long to work out. It wouldn't take long. This is likely to have little impact. Mor importantly large, commercial aircraft are very unlikely to just cancel IFR out of no where.
This one would be easy to see. The antenna for omni-directional broadcast for a long range is quite large, and to have an impact you would need to broadcast your gibberish on a very large number of frequencies. Those two things together would certainly make your stand out in your neighborhood.  The antenna may not look big at the airport, but it will in your back yard. Most likely you won't even finish getting it build before someone comes to take a look. If you do, then you will be found very quickly. Broadcasting a signal gives law enforcement a beacon to home in on. Don't forget ADF did just that for many years.
Fake weather would be pretty hard. Re routing and such may also not have that large an impact. Lets not forget you have to convince the pilots that your actually giving them a valid route. Depending on who your impersonating, that could be interesting.  For example, at KMIA there are SIDs/STARs, you could give out the wrong ones of thous, except, at such a busy airport everyone already knows their SID/STAR before they even take off. In fact, even at smaller airports, SIDs are tied to destination directions and STARs are tied to arrival vectors. Try giving a south bound STAR to a northbound plane, and the pilot won't likely be able to comply, and will ask for clarification. Messing up spacing or timing might be a bit easier, but see point 2. You pain a giant bulls eye on your back, your not going to last long.

Bonus - Comms failures are not unheard of, there are other ways to communicate with ATC if you need to. Most of a flight, even an IFR flight, you don't really talk to ATC much. You gain clearances and switch "areas" (generic) but other then that, it's pretty quiet. It only really gets loud around airports or busy "areas". And with both sides of the conversation being able to hear anything said, both sides can go "nope wasn't me". And when in doubt the pilot just pilots. If it''s so bad that ATC goes dark or the pilot can't possibly make out what's going on. They just fly their last route/clearances/flight plan.  Your going to have a very hard time getting a pilot, even a new one, to make a course change that puts his plane at risk. Avaite, Navigate, Communicate. That communicate one is last for a reason.


text:
What are these holes in the engine inlet?
In the next picture can be seen within the housing, near the inlet:

Are those definitely holes and not rivets? Also what are "aerodynamic takes"?

Sorry, I 'm Spanish and my English is pretty bad. " aerodynamic Takes" is the literal English translation of " toma aerodinámica" in Spanish , which is what you call in English " inlet" .

Is there any way you can put a circle or an arrow on the photoindicating specifically what you're referring to?

It is done. Now that I look, yes they could be rivets...

My first reaction is rivets, comparing them to the rivets on the seam to the right. The circled ones do look a bit randomly spaced for rivets, though. I'm wondering if it might be the textured surface of the acoustic liner. Hopefully someone that knows for sure will answer.

Rivets doesn't appear always evenly spaced or aligned.

It is very possible that they're holes too. For example in CF34-10E5 engine the nacelle inlet is full of small holes. Unfortunately I have no idea what they are there for.

Could serve to absorb the boundary layer?

@Sami the many small holes further downstream in the inlet serve to reduce the noise.

My eye immediately went to the 6 dark spots on the spinner, which could be holes, and thought "That's a good question!". I guess I was looking at the wrong holes.

The holes on the spinner are actually for access to the bolts that hold the tip of the spinner in place. You remove that plate to get inside the spinner.

Here's a picture of how the spinner is attached .http://imgur.com/a/lWLHq

the circled points are definitelly rivets, they are pretty smooth and flush with the inlet. No ideea about small holes for noise, but the IAE engines we use have no such holes before the fan blades

They look like rivets to me.

These are "acoustic liners"; they allow the sound waves to get "trapped" in the holes and make the engine quieter. Sometimes chunks tear loose and they're repaired with what amounts to Bondo.

So it's like a silencer for jet engine hah, pretty cool

This is anti-ice system. Hot air is blown in this holes and ice/snow not settle in jet engine.

Welcome. Can you provide a reference to this?

No. The silver cowling is heated for anti-icing. These holes aren't for heating, but as @fooot commented above, for noise reduction.

This answer is wrong.

The holes you are referring to are a part of the engine-inlet anti-icing system. These holes prevent ice accumulation around the engine inlets. Without them, the engines may ingest ice and will also encounter airflow problems. Engines require laminar airflow in order to operate efficiently.

Respectfully, your answer is incorrect.

authors: Дмитрий Емельянов


text:
What is this F-18 Hornet "air intake" for?

I think I read once that they served to cool the on-board computer systems, but I'm not sure.

Related: https://aviation.stackexchange.com/q/38140/15311

It's for keeping the boundary layer flow from entering the intake, it's called a splitter plate. I think it's done to prevent turbulence inside the turbine (prevent compressor stall) and to make sure the intake doesn't ingest slow air, to maximize efficiency.

I'm guessing this is because the intakes are so close to the fuselage that they have this. Otherwise, the boundary layer shouldn't be an issue since in most other aircraft, the intake is much further away from the fuselage. Am I right in this thinking?

@ironduke97 I think yes, you‘re correct.

Have a look at the intakes of the Eurofighter or the Dassault Rafale, they have similar plates between them and the fuselage.

The upper and lower arrows point to the air-path of the boundary-layer splitter. But the OP's middle arrow does point to an air intake - anyone know what that is?

@RAC that's the port that releases the chemtrails of course

Ahh, the infamous chemtrail inlet port. By the way, does anybody have video of one of these (or similar) on full afterburner where you can judge scale by sighting the horizon? I am pretty sure that is what chemtrails look like when you light them up.

Everybody knows chemtrails are real... it's just the crazies that fail to realize that water is a chemical. ;)

EDIT: I'd just finished writing out my own answer when I stumbled across this answer to a different question, which nevertheless explains the function of the small intake in greater detail. I've included my answer below anyway.

A couple of people have mentioned the splitter plate, but failed to address the smaller intake set between it and the fuselage. It's very difficult to find any references to this in particular, but my first instinct was that it's probably to do with either engine cooling (Concorde has additional intakes beneath the engines for this purpose) or engine bleed air.
After a bit of reading I resorted to Wikipedia, and found the passage below:

... the Hornet uses bleed-air vents on the inboard surface of the engine air intake ducts to slow and reduce the amount of air reaching the engine.

So it seems to be a simple intake for bleed air which has the additional effect of slowing the air entering into the main intake. Incidentally, this bleed air is redirected to a slots just above over the wing, to improve the performance of the aircraft.

Bleed air vents allow air to leave the main duct, i.e. they are outlets, not inlets.

@Jan Hudec The slots above the wing are the bleed outlets.

JF-17 has something like this on top of it's right main intake. But this small, secondary intake is found only on one side. https://www.strategicfront.org/forums/attachments/fitsxrovcaezmqr-jpeg.22440/

The intake is for the primary heat exchanger. secondary is on the other side, Same location. The ambient air flows across a large rectangular heat exchanger.  Basically a air to air radiator for the Bleed air system used for most functions on the F/A-18. Air conditioning, Wave guide pressurization, cabin pressure, avionics cooling, Gun,Etc. It cuts the air temperature in half and water is sprayed on the face of the heat exchangers that is extracted in the cooling process. Aids in cooling especially while sitting on the deck at idle. Otherwise you get a AVAIR hot code. The ACACS system has a water separator in it. That's where the water comes from. The Spent air and water exits on top of the aircraft. Basically without this system the Bird wont fly! If you ever see a F/A-18C throttle up on the deck you will see the water exit on top. It appears to be smoke but it is a water mist. I hope that answers some of the questions.

Welcome to Av.SE! Did you fly the Hornet?

So the water is evaporated and discarded? What happens if you run out of water?

The rectangle in the center is the housing for the primary heat exchange.  There is another heat exchange on the right side.  The two "ramps" on top and bottom is to bleed off air during transonic/ supersonic flight.

If I remember correctly, bleed air from those tiny intakes is used for defogging the cockpit, blowing air across the front of the canopy to increase visibility under heavy rain, and pressurising the external fuel tanks.

As suggested above it not an intake but rather a aerodynamic plate that keeps the boundary layer out of the engine. This is normally required for supersonic flight since the engines have to carefully manage the shock waves entering the engine. Most engine designs are not capable of operating under supersonic conditions and require that the inlet is carefully designed to create a series of shockwaves that slow down the air to an appropriate Mach number before it reaches the compressor.

Ok, but just like the other answer, this misses to give an explanation for the particular intake at the tip of the central arrow. What is that intake and where does the air sucked in there go?

you may not use "above" to refer to previous answers as answers may be reordered depending on their upvotes even several month after being written. You can use the permalink instead.


text:
Why don't airliners have any mirrors or cameras?
It seems odd that airliners do not have an easy way to monitor critical components on the exterior of the aircraft, should for example, a bird or something else strikes hard enough to cause damage. A mirror or camera would allow a pilot to see and react to a possible dangerous situation quickly. Also, a camera helps a pilot to see what's behind.

Related: Which aircraft feature stock exterior cameras?

See this answer (Periscopes on the VC-10) for something in this direction.

Aircraft rarely collide with any objects. Cameras would only be helpful if the extent of the damage is visible from the outside. Instruments, on the other hand, can be used to diagnostic the problem even if the system fails "internally".
E.g. Many hydraulic lines and cables run across the wing. If these is damage on the leading edge, is it the slats? A hydraulic leak on system A? system B? Is the de-ice working? You need instruments to get a better picture.
Some aircraft indeed come with cameras. The Boeing 777 has cameras on the bottom of the fuselage and the tail to aid pilots during taxi operations (the 777, especially the 777-300 variant, is really long...)

The Airbus A380 also has a tail mounted camera. Passengers can enjoy a spectacular view of the clouds, the high speed takeoffs and landings:
As for mirrors, pilots don't overtake other planes or back off like drivers do. There really is no need to know what is behind the aircraft. Pushback is accomplished by the tug on the ground, who has a much better view of the area than the pilots.

Also, I guess if you see a blot from a bird strike, you almost only see damage to the bird and can hardly recognize any damage to the aircraft...

As for avoiding crashes, planes have radar. At the close in speeds typical of in air collisions its probably too late when you see them in the rear view mirror...

@Aron Commercial airliners do not have radar for avoiding crashes. They rely on TCAS to provide adequate collision warning.

@MichaelKjörling 1) I suspect this A380 is on the ground and lining up to takeoff (the A380 tail is huuuuge) 2) You might want to see the nose aligned with the runway but on a crosswind landing the pilots don't, until the last second anyway.

I've flown on the AirFrance A380, and there were multiple camera angles you could toggle through on your seat display. I'd say 5 or 6, Top of Tail Forward, Under the fuselage, Right Wing Out, Left Wing Out, and Nose.

@MichaelKjörling: Try landing a plane in Kerbal Space Program on a computer with just 4 GiB of RAM.  Touching down on the runway at all is something of an accomplishment.


text:
What is a scab and why is being one so bad?
I understand that a generally speaking a scab is someone who crosses a picket line. In aviation, however, there seems to be severe opposition (hatred?) toward flying flying with a scab, to the point of swapping legs. There also seems to be some pretty strong public shaming through publication of personal data.
So: What is the origin of this stigma and why has it become so bad?
(Caveat: I have no experience with unions, so this may be entirely normal and I just haven't had the exposure. If that is the case, please indoctrinate me.)

This is nothing to do with aviation.

The question hints at something like "is their reputation especially bad in aviation," which could be on topic, but that sounds like it could be too opinion-based.

Mainly language related and potentially opinion based

A scab is a strikebreaker, an individual hired by a company to do work performed by unionized workers that are on strike, or a union member who rebels from his union and goes to work while there is a strike, crossing his own picket line.
In an airline context, if there is a pilots' strike, some pilots who are union members may decide to cross their own picket line and go against their union in order to keep flying, especially if they feel the strike is unjustified, or they are financially on the edge.
As emotions are usually high during these sorts of things and a lot is at stake, those who cross are seen as "traitors" by the rank and file who made the sacrifices you might say.  After the strike the pilots who went out and the pilots who crossed still have to work together, but the ones who went out will be very resentful of the ones who crossed.
Most will just sort of let it go and carry on, but there are always hotheads who will want to punish the rebels, hence the personal sanction campaigns, intended to make the picket line crossers think twice about doing it again.

First a general answer, and then specifics
The problem with the "scab" as seen by a union member - during a labor dispute between management and the {pilot's} union - is that the "scab" is directly undermining the leverage and bargaining capacity of the  union that has a work stoppage as one of the tools available for negotiation of a wide variety of things.
This is consistent with union activity going back about a century in the American labor movement.  (My answer can only really speak to American labor movement.  I am not sure how this fits all other countires, but my limited experience in Italy 20+ years ago is that their unions were generally much stronger).  My dad was a union member for a few years in the steel industry in the Western Pennsylvania.
The disputes that come to that point that leads to a threat - stoppage of operations - are usually the result of management and the union coming to an impasse that needs to be overcome in order to arrive at a new contract/collective bargaining agreement/pay scale, whatever the issue is.
The "scab" pilot (or a whole bunch of them) give management a get out of jail free card rather then keeping negotiation going (in good faith or bad).  That means that the "scabs" have taken money out of the pockets of all union members (or so a union member's perspective will often go), not only in the short term but in the longer term for all members of that union if it gets the union to back down and accept a lesser settlement.
That has serious long term implications on quality of life for thousands of pilots if it breaks the strike.
Yeah: some long term ill will can be accrued.
Specifics from my life experience:
When Frank Lorenzo was engaged in union busting with Continental in the 80's, there was some bad blood between some of the recent hires and those who had been there for a longer time.  I knew people in both camps who were reservists who flew with us (I was in the Navy, they were in the reserve unit supporting our squadron).     What I saw, in person, was the "we don't speak with X" played out in our squadron over what was going on at Continental.
I won't start with the Teamsters, with whom I had a very small amount of RL experience.  I'll just say that shunning is a powerful peer influence tool.
Aside:  My father has some rougher stories about people (scabs) who got beaten to a pulp for crossing a picket line back in the 40's.


text:
When does the FAA require OEM involvement for the approval of an STC?
There appear to be many instances when companies modify aircraft (or introduce after-market systems) without the involvement of the original equipment manufacturer (OEM), up to conversions of passenger aircraft to freighter aircraft.
Is there guidance from the FAA on when an aircraft modification requires the involvement of the OEM?

It tends to go on a case by case basis.  OEMs tend to steer clear of STC developers because OEM involvement tends to drag the OEM into an implied commitment to support the STC itself, or support the STC's secondary effects on how the airplane is operated, that the OEM would rather not be stuck with.
What will happen is an STC developer will submit a certification application with documentation, test reports and all that, to the regulator, and most of the time the regulator will approve the STC independently of the OEM.  But there may be cases where the regulator's engineering group will demand OEM engineering input, or at minimum an "NTO Letter" (NTO = No Technicial Objection) before approving the STC.
In such a case the ball is in the OEM's court and if the OEM insists on not getting involved, well the STC developer is stuck and the project might die on the vine unless they find some other work-around.
In other cases however, the OEM may actively support an STC application where they would like to see the mod go into service because there is a benefit to the OEM.  You might see this for a mod that improves fleet reliability, or is just really popular with operators, but where the OEM lacks the internal resources to develop it in-house due to budget limitations.


text:
Why are some parked aircraft angled away from the taxiway center line?
With all the aircraft parking going on these days images like this one have made me wonder why the aircraft aren't just parked nose-to-tail:

I realize that rotating the longitudinal axis can help in cases where you move the nose of one aircraft past the tail of the next aircraft. But most of the time the aircraft in the image above are far enough apart for that to be a benefit.
Here's what seems to be a more efficient example:

One thought is that it might be due to prevailing winds on the airfield? Or is this just specific to the A380 or the location?

If the pavement is wide enough and strong enough (both of which will vary depending on the model of plane) for staggered parking, then that is the most space-efficient option.
If not, then you have to park them single-file. Simple geometry tells us that turning planes roughly 45 degrees (◼◼◼) will be more space-efficient than nose to tail (). For the same reason, some very congested airports have tighter angled parking at gates, whereas those with plenty of room use faster straight-in parking.


text:
Did the Doolittle raid run their engines over square?
My flight instructor made a comment when talking about complex airplanes that "never over-square an engine" is a good rule of thumb but secondary to POH instructions. The POH, (I presume) is the best-informed source of how the engine was intended to operate.
He then went on to give an example: Supposedly, since the Doolittle Raid had to launch their B-25B Mitchels 200 miles further out to sea, they ran their engines over square which changed their fuel consumption such that they were still able to pull of the mission.
So here are my questions:

Did the pilots during the Doolittle raid run their engines over square?
By what mechanism would running over square change fuel consumption that dramatically? Is that unique to the B-25B's engines?
If there is such a big fuel consumption benefit, why is the "never over-square an engine" rule so prevalent? (Seems like this competes with operating the engine at "best economy"?)

Image of B-25B launching off the USS Hornet:

Did you read the Aviation.SE answer you linked? "Over Square" is a myth, a well-believed one at that, but a myth nonetheless.

"Never over square" is a handy rule of thumb because if you stay below square, you don't need to read the POH or remember what exactly is allowed for each plane. If you do, though, over square can be just fine.

@RonBeyer - yeah, I originally had the question written more negatively, but opted to maintain civility... :)

According to this transcript of the actual mission report from one of the pilots, they did indeed run over square

I flew the last 500 miles into Tokyo at altitudes ranging from 1000
  feet to 4000 feet in order to fly in the thin overcast and clouds to
  avoid detection. I used 28 inches of manifold pressure and about 1370
  RPM and indicated between 150 and 165 MPH.

And for the latter part of the mission

I made the trip from Tokyo to China at about 500 feet altitude and
  1300 RPM and started at 29 inches of mercury and gradually reduced to
  25 inches as my gas load reduced. I indicated about 160 to 165 MPH.

Question 2: According to this copy of the manual (page 155) the engine is almost always run over square in cruise.

As for question 3 you can find a discussion on operating over square in this question and ill avoid answering it here to keep it form being a dupe.

The whole "oversquare" thing is a myth based on peoples' perceptions, not reality.  They are just numbers.  As long as detonation isn't happening, you can run at any MP/RPM combination you want.  When you shove the throttle to wide open from an engine that is idling, you are "oversquare" to the blazes for a few seconds aren't you, and by rights the engine you detonate itself to pieces right there, but it doesn't.
For best fuel economy, you want RPM as low as possible, while still running smoothly, with temperatures in limits, and producing the required horsepower, to minimize friction and heat losses in the engine.
This favours low RPM/High MP settings and also extends engine life.  Bottom line is, the slower the piston rings are travelling up and down the cylinder bore per mile travelled, the better.


text:
How can I tell a GPS installation is acceptable for IFR use?
I am evaluating an aircraft for use under IFR.  I want to know if the GPS that is installed is acceptable for use under IFR.  I have access to the logbooks and documentation.  I want to check:

That the GPS unit itself is acceptable
If the installation is acceptable
If the database is current

Operational considerations and limitations may be discussed, but are secondary to the scope of this question.  Citations to the AIM and AC 20-138 should be in a bulletized summary.

Which GPS is it?

Should not matter.  Ideally an answer would tell me where to look to see which TSO a unit was manufactured under and what magic logbook words are that show me the installation meets the requirements of AC 20-138 and STC requirements.  Checking the database is pretty easy.  Extra upvotes expected if there are things that I have not thought of.

The easiest way is to read the Aircraft Flight Manual (AFM) or Aircraft Flight Manual Supplement (AFMS). If the the GPS was installed at the factory, it will be addressed in the AFM. If it was installed later via an Supplemental Type Certificate (STC), there must be an AFMS to cover the operation of the GPS.
If the GPS installation was not at the factory, there should be an a/c logbook entry showing the installation and STC approval.
As specified in AC 20-138D:

19-9. Operations manual or AFMS/RFMS.
An AFMS/RFMS must be provided that contains the limitations and
  operating procedures applicable to the installed equipment
  configuration. The AFMS/RFMS must address the operation of the
  equipment and the related components as they are installed (e.g.,
  remote source selector). An AFM/RFM supplement is required if the
  installation is based on a prior approval that had an AFM/RFM, or if
  there are any limitations associated with the operation of the
  equipment.

The key information is covered by the first sentence -- limitations and operating procedures. If the GPS is not approved for IFR, there must be an operation limitation in the AFM/AFMS that states that (and in all of these cases I've seen, the FAA also required a placard stating "GPS not approved for IFR flight.")
If the information is not included in the AFM or AFMS, I would question the approval of the installation as all installations require the AFM/AFMS.
As for the database, it expires every 28 days. Turn the GPS on. If the database is not present or expired, you will get a message to that effect.

Note that there are often multiple databases configured with a GPS unit. There may be one or more of: basemap, terrain data, obstacles, instrument approach procedures, STARs, SIDs, radio frequency names (e.g. to associate with an airport), other navigation waypoints, user-defined waypoints, etc. Each of these will have a revision date and/or expiration date, and the map provider's website will provide the current information. In some cases, you might obtain some information, such as the terrain database, from the manufacturer, and instrument approach procedures from a provider like Jeppesen.

It has to be a unit that meets TSO (Technical Standards Order) 129, or for newer units that can use Wide Area Augmentation for LPV approaches and has other features, TSO 146 (the significance of each TSO and how they apply, you can look that up).  That'll be in the unit's specification in the operating manual.
Installed and signed off by an approved avionics shop in the log entry.
There will be a database information page in the unit that will show the expiry date of the installed database.  The manufacturer's website should also have a listing of its database library with dates etc.


text:
During instrument approach, is an abbreviated racetrack course reversal allowed?
I was flying the RNAV (GPS) RWY 33 approach into KFNL during instrument meteorological conditions.  The plate identifies a non-standard 6 NM racetrack reversal at IMOMY:

Denver approach cleared me direct to IMOMY.  I was approaching from the Southwest, but NOT from HYGEN. Thus, I began a circuit around the depicted procedural track. I was cleared for the approach and approved to change to advisory frequency.
At about 4 NM outbound, ATC contacted me* and asked when I planned to turn inbound.  I advised that I could begin inbound immediately, which they then asked (not cleared) me to do, in order to avoid encroaching on departing traffic from KDEN.
During VFR practice approaches (not under ATC), we always shortened the course reversal on this particular approach to save time. When doing the real thing, it was always my understanding that the depicted track had to be flown EXACTLY as published.  Per the FAA Instrument Procedures Handbook:

A holding pattern in lieu of procedure turn may be specified for course reversal in some procedures. In such cases, the holding pattern is established over an intermediate fix or a [final approach fix]. The holding pattern distance or time specified in the profile view must be observed. For a hold-in-lieu-of PT, the holding pattern direction must be flown as depicted and the specified leg length/timing must not be exceeded. (Page 4-53)

Now, I was also taught that if you have to perform a teardrop or parallel entry, that completed your reversal and no circuits are necessary:

The holding pattern maneuver is completed when the aircraft is
  established on the inbound course after executing the appropriate
  entry. (Ibid.)

Question 1: Was it appropriate for me to attempt to fly the 6NM circuit?
Question 2: Am I allowed to choose an arbitrary leg distance (or time), as long as the 6NM limit is not exceeded?
*I was already switched over to the CTAF, but still monitoring approach control on the second radio which I always do just in case. Something I picked up from my flight instructor who flew larger planes for a living...

Unless you were "cleared straight-in" for the approach, when IMOMY is your IAF (as it was in this case) you are required to fly the course reversal, even if you can execute a direct entry. This is because the approach does not have terminal arrival areas labeled NoPT. So, yes, it was appropriate to fly the 6nm reversal.
That said, no, you cannot choose a different course reversal, even a shorter distance hold. When a hold is depicted in lieu of a procedure turn (HILO) you are required to fly the hold as shown.
AIM 5-4-6 (4) states:

If proceeding to an IAF with a published course reversal (procedure turn or holdinlieu of PT pattern), except when cleared for a straight in approach by ATC, the pilot must execute the procedure turn/holdinlieu of PT, and complete the approach. [emphasis mine]

The IFH that you cited in your question has similar wording:

For a hold-in-lieu-of PT, the holding pattern direction must be flown as depicted [emphasis mine]

Occasionally ATC will act surprised or get frustrated with you for flying a reversal when you're within a 90° turn of the initial segment's course, but unless they specify straight in you're doing the right thing.

I believe the part of this about having to fly the full distance is mistaken. See https://aviation.stackexchange.com/a/84457/40747 and the linked letter from the FAA.

To reproduce the gist of the content from the answer linked to above, the FAA letter of interpretation is Young-HHC-2011 stating: "it is permissible, without specific ATC clearance, to shorten published outbound DME legs in a holding pattern".

@JeffB The link the Young HHC letter has been updated to https://drs.faa.gov/browse/excelExternalWindow/FAA000000000LEGALINTPR2011100PDF.0001 . The link you use results in a 404 error. for anyone wanting to quickly reach the specified question search the document for "Question 5"


text:
What role does this ground crew member fulfill?
At the Miramar airshow a couple of weeks ago I noticed that in addition to the crew chief of each Blue Angel there was one additional individual standing behind the crew chiefs between aircraft #3 and #4:

What role does that individual fulfill?

Head crew chief?

Looking through the Blue Angels team roster these are most likely the "Crew Coordinators".


text:
What does a Letter of Intent contain?
When operators purchase new aircraft it seems that a Letter of Intent is frequently the means by which this is done. What does such a Letter of intent usually contain?
Bonus question: Would the letters of intent in the following two scenarios would look different:

Purchase of a new aircraft as the launch customer from and established manufacturer. For example: ANA was the launch customer for the 787, which was a new type certificate from company with an established track record of manufacturing airplanes.
Purchase of a new aircraft as the launch customer from a new(er) manufacturer. For example, Continental Express purchased 25 ERJ-145 at a time when Embraer was not an established manufacturer of commercial passenger aircraft.

Would the purchases in scenario 2 expect more/better assurances as part of the LoI  to minimize certain risks (e.g. planned partners, production planning, certification planning, etc.)?


text:
How much does the design, implementation, test and certification of a Passenger-to-Freighter Conversion cost?
This question answers how much it costs to convert a given aircraft from passenger to freighter once the STC is approved by the FAA. How much does it cost to obtain the STC in the first place?
If this data is not publicly available, would there be a generic way to estimate the cost of such a conversion?

Related Question:
From what I've found the required design and analysis is primarily focused on the modification of the aircraft structure (as opposed to e.g. flight controls), and to some extent related to the avionics that monitor the health of the various onboard aircraft systems (e.g. ensure that no warnings/errors are displayed to the flight crew as a result of the absence of the passenger entertainment system).
Does that imply that the cost to generate the certification basis for the initial conversion would be fairly comparable across different aircraft? I realize that the labor of the actual conversion (once approved) will differ based on the size of the aircraft.
By way of example - how much would the required analysis differ between the cargo versions of the CRJ-200 and the B737-800BCF?
Cargo Version of the CRJ-200:

Cargo Version of the 737-800:

Bit of a "how long is a piece of string?" question perhaps? Depends on the size of the aircraft, air-frame materials, intended cargo... ...can you be more specific?

That's kind of the "related question". If you're going to add a cargo door to a CRJ200 or a 737-800 that fits your traditional ULD cargo containers (e.g. LD-3, ASZ, SAA, etc.), does that work generalized across aircraft. By comparison: a former Honeywell DER once explained to me that a Fly-By-Wire flight control system is ~5000 requirements that need to be traced, verified, coded, and tested, no matter what aircraft if goes into. Can the same be said about structural modifications such as a P-t-F conversion?
Also, "how long is a piece of string?" <-- great expression!


text:
If the "Fuel Shutoff Valve" is ON, wouldn't fuel be cut off?
Ok, so this may be more of an english lanugage question, and I can confirm from context that when the fuel shutoff valve is ON fuel does indeed flow to the engine, but wouldn't that just make more sense to call that valve the fuel valve?
If the component that shuts fuel off is on, operational, in use, functioning, etc., shouln't fuel be shut off, as the name implies?
I realize this could be seen as a quibble, but confusion causes accidents and IMO this is wildly confusing.
EDIT
Looking at the Cessna 152 checklist today I noticed this interesting quirk. On the regular checklist side (pre-takeoff, maneuver, etc.) the checklist reads "Fuel Shuttof Valve - ON", but on the emergency checklist for an engine fire it merely states "Fuel Valve - OFF".

It depends on what word is being operated on. The valve is the "Fuel Shut-off Valve", and the fuel is either "On" or "Off", not the valve. This doesn't nessesarily make the most sense from an english language perspective, but from a UI sense, people will see Fuel, and they will see On or Off. This is probably what the first engineers were thinking. In an emergency, you want people to press the button that says Off, so it makes sense to label it as such.

From a UI sense I agree it makes sense. From a UX perspective I think we should call it the "Fuel Valve"

Check out this bit on competing standards

Talking about confusion - French/European (eg Cessna) made switches are off in the opposite way to American (eg Piper) aircraft. Flying both regularly I cant tell you how many times this has made me leave master switch on!

In the small Cessnas, the fuel valve is located under the panel above the pilot's right knee. It is pushed in to be open, and wired in place with soft wire that can be easily pulled to close the valve in an emergency. So forward is open, just like forward for full throttle, forward for full rich on the mixture, and forward on the prop for full power (not applicable the 150).  "Balls to the wall" for everything for full power!

I can see where that can be confusing, but it comes down to perspective.
From a maintenance and engineering perspective, you are actuating the fuel shutoff valve which controls the fuel to the engine.  The actual terminology when referring specifically to the valve would be opened or closed, which would result in the fuel being on or off.
From a pilot perspective you are simply turning the fuel selector on and off, which happens to be connected to the fuel shutoff valve.  The pilot doesn't even need to know about the valve, so they placard it from the operational perspective in the cockpit and say whether the fuel is on or off.

Note that these are placarded as a fuel selector and fuel off both of which seem pretty clear to me.

I'd think that 'fuel shutoff valve' is a good name, but I'd think it's more sensible to use alternative terms to on/off. I think however most pilots will very early learn this and that it can't really be a problem. They know which position it should be in.
A few aircraft I can think off will avoid this by only using a using a more expressive term than on or off. For instance: OPEN-ON or likewise CLOSED-OFF or even more simply, OPEN or CLOSED. Some also explicitly state the difference, for instance 'PULL-OFF', like the aircraft below.

(source: capnhq.gov)

Shutoff has a negative connotation, but the word ON is (highly) associated with the engine running. So if SHUTOFF is ON, is the shutoff valve operational (cutting the fuel off) or do we run with our standard interpretation of the word ON - that something essential (fuel supply) is functioning? I can see a pilot in an emergency, when her or his brain is on the edge of panic, forgetting their training and operating the valve incorrectly. I'd suggest it would be clearer to keep the word Shutoff, but make it one of the options, rather than the name of the whole control.

(source: massey.ac.nz)

emergency, when her or his brain is on the edge of panic, forgetting their training - No.  An emergency is exactly when you won't remember anything except your training.

@SteveV. Not quite correct either. No matter what do you remember, you act  by reflexes. For most people, combination of FUEL and OFF is read as "this selection stops fuel" before they actually read, process and undestand the text. Therefore such signs should be well thought out: OPEN and CLOSED from the other answer seem to be a good choice.

Steve V and I agree: in emergency situations, the cognitive functions of the human brain decrease. In such situations, eliminating the potential for confusion is paramount. In an engine fire, the pilot’s primary problem is that fuel is getting to the fire, not the state of a valve, so go for words whose association with flow is unambiguous. Many pilots have electronics somewhere in their background and – under pressure – could well interpret OPEN and CLOSED to mean flow-inhibited and flow-enabled respectively. ON and OFF do not suffer from that problem. (and OFF is even simpler than SHUTOFF)